1. Technical Field
The present invention relates to a water purification process and more particularly, it relates to an improved process for the desalination of salt water by the strategic use of ion selective membranes to form a variable make-up that is fed to a desalination unit to produce a water product of potable quality.
2. Description of Related Art
The basic processes for the desalination of salt water to produce a water product of potable quality include, for example, multistage flash distillation, multieffect distillation, reverse osmosis and vapor compression distillation. Each of these processes are well established technologies having their own unique characteristics and limitations. The high content of scale forming ions, saline and other impurities found in untreated salt water has a negative impact on the efficiency, energy consumption and maintenance of large scale plants which employ any of these conventional desalination processes. Due to such high concentrations of impurities, it is commonly known to add antiscalling chemicals to the feed or equipment to minimize the adverse consequences of scale forming ions.
Multistage flash distillers which are in common use worldwide for large scale desalination projects are limited in their performance by the maximum brine temperature (top brine temperature or TBT) that can be used in the process. At temperatures higher than the maximum, scale inhibitors are ineffective and significant fouling of the internal surfaces will rapidly occur. This can be expensive and time consuming to remove.
Membrane technology has been used in the pre-treatment of salt water to reduce the high ionic content of salt water relative to fresh water. For example, U.S. Pat. No. 4,723,603 discloses a process for removing precursor ions from an injection water which formed insoluble salt precipitates in situ when they contacted resident ions already present in a subterranean hydrocarbon-bearing formation. The precursor ions of the insoluble salt precipitates are removed by means of a RO membrane.
More importantly for desalination, WO 99/16714 discloses the combination of membrane technology with the basic desalination processes to from a drinkable water product. According to this document, saline water containing a high content of hardness scaling ions is passed through a nanofiltration membrane to form make-up to a desalination system. Nanofiltration softening membranes are used for the selective removal of hardness scale forming ions and other impurities to soften seawater. Due to nanofiltration treatment, the make-up has a reduced ionic content when it passes through the desalination system. It is reported that there is a reduction of scaling and fouling tendency when this combination of nanofiltration and desalination systems is employed. However, the document does not suggest any means of taking advantage of the inter-relationship among certain conditions, e.g., pressure, temperature and make-up, to achieve an optimal system and recovery.
It will be seen that in spite of these disclosures, there is still a great need for a process that optimizes the combination of these two technologies to improves the operating conditions, efficiency and yield of this type of hybrid desalination systems.